The present invention relates to photolithography methods and, more particularly, to optical lithography methods for forming fine-sized patterns on a wafer or other substrate, such as using a photolithographic mask and a projection lens.
In existing projection systems used in optical photolithography, a quasi-monochromatic, spatially incoherent light source of wavelength λ is used to illuminate a photolithographic mask having various patterns, such as a periodic pattern of equally spaced lines. The illuminating beam is usually collimated to ensure a highly uniform intensity distribution at the plane of the mask, and an adjustable condenser stop is used to control the degree of coherence of the illuminating beam. The light is transmitted through the mask and collected by a projection lens which images the mask patterns onto a wafer located at the image projection plane, typically at a predetermined reduction ratio.
In such projection systems, a lines and spaces pattern on the mask diffracts the illuminating beam and forms a plurality of light beams that pass through a projection lens. An optical image of the lines and spaces pattern is formed on the wafer when the light beams interfere with each other. The smaller the pitch of the lines and spaces pattern on the mask, however, the larger the angle at which light diffracted by the mask spreads. Thus, if the pitch of the lines and spaces pattern is sufficiently small, the angle defined by two adjacent diffracted light beams is large enough for the first order and higher order diffracted light beams to impinge outside the projection lens so that no optical image is formed on the wafer.
To print such smaller lines and spaces patterns on a wafer, projection lenses having larger numerical apertures are used to accept larger incidence angles of diffracted light. The numerical aperture (NA) of a projection lens is defined as NA=sin θ, where θ is the half-angle of a cone that is subtended by the clear aperture of the projection lens at the wafer. As an alternative, the exposure wavelength is decreased to decrease the angle of diffraction occurring at the mask. In both methods, however, as the lines and spaces patterns that are to be printed approach submicron sizes, the contrast of the patterns formed on the wafer deteriorates, and the depth of focus decreases. As a result, neither alternative is practical at these smaller dimensions.
To form finer lines and spaces patterns without sacrificing contrast or depth of focus, a phase-shifting mask is used. The optical phase of light transmitted through some or all of the mask is changed by changing the thickness of the transparent regions of the mask, either by depositing additional transparent material where needed or by removing a thin layer from the mask at specific locations, thereby selectively adjusting the transmitted optical phase at these locations. Using the phase of the light, the phase-shifting mask eliminates the zero-th order diffracted light beam, namely the light diffracted by the mask pattern in a direction parallel to the optical axis of the projection lens, which would otherwise cause deterioration in the contrast. Only first order diffracted light beams, which are generated in directions symmetrical with respect to the optical axis, pass through the projection lens and interfere to form the optical image on the image projection plane. As a result, the incidence angle of the interfering light beams can be the maximum angle of incidence of the projection lens, thereby increasing the depth of focus and allowing for the printing of finer lines and spaces patterns.
A further alternative using a projection system is described in U.S. Pat. No. 5,636,004, titled “Projection Exposure Method and Apparatus” to Ootaka, et al., the disclosure of which is incorporated herein by reference. A conventional chrome-on-glass mask having a lines and spaces pattern with a pattern pitch 2L, where L is a value between 0.5 λ/NA to 1.0 λ/NA, is illuminated with vertically incident light to form zero-th order, first order and higher order diffracted light beams. The zero-th order and first order diffracted light beams pass through a projection lens and interfere with each other on the image projection plane to form an optical image on the image projection plane. The wafer is first exposed at a distance z from the focal plane of the projection lens, and then the wafer is exposed again after the wafer is moved along the optical axis by a distance Δ, where the value of Δ is chosen such that a lines and spaces image having a pattern pitch L is formed on the wafer by the interference between the +1st-order (positive first order) diffracted light beam and the −1st-order (negative first order) diffracted light beam without any dependence on the value of the defocus distance z.
The above methods, however, require that wafer be illuminated with vertically incident light, also known as axially incident light. As an example, if the method described in U.S. Pat. No. 5,636,004 is carried using non-vertically incident light, the intensities of the +1st-order diffracted light beam and the −1st-order diffracted light beam will not be of the same intensity so that the two beams do not interfere completely and will degrade the pattern formed on the wafer.
It is therefore desirable to provide a method for forming fine-sized patterns on a wafer without requiring that the light be vertically incident.